This invention relates to techniques for measuring the mass of liquid in a storage tank, and more particularly to a method and apparatus for measuring the amount of liquid fuel in the fuel storage tanks of a spacecraft.
In many situations, it is necessary to be able to accurately gauge the amount of liquid remaining in a storage tank. When an undesirable event accompanies the exhaustion of the liquid, it is particularly important to be able to estimate the remaining liquid accurately.
One example of where all the foregoing exist is in a satellite attitude control system employing gas jets or thrusters. Gas for the jets or fuel for the thrusters is usually stored in liquid form in containers or tanks on the satellite. When the gas or fuel is used up, most satellites become inoperative, or at least suffer significantly degraded capability. Moreover, without gravity to pool the liquid in the tank it tends to distribute itself in response to other forces. Fuel is generally used herein to denote the fluid contents of the container, whether or not the liquid would strictly be considered fuel.
The length of time that a particular load of fuel will provide stationkeeping for a satellite cannot be predicted exactly, because it depends upon environmental conditions, such as solar wind pressure and magnetic fluctuations, and also on thruster performance. Furthermore, the exact amount of fuel used during each maneuver also depends upon the exact characteristics of controllers, which may vary slightly with time and environment. Thus, the exact amount of fuel remaining in a spacecraft becomes less well known as time progresses, unless a suitable measurement scheme is used.
At the end of life of a geostationary satellite, for example, the remaining fuel is used to remove it from its assigned orbital position so that a replacement satellite may be inserted into that orbital position. So long as the old satellite is given enough velocity to vacate the regions of interest, it is irrelevant how much additional velocity it is given. If the amount of fuel remaining in the tanks at the time the satellite is ejected from its orbital position exceeds that necessary to send the satellite to the graveyard of old satellites, the excess fuel could have been used for additional useful time in orbit. Because the amount of fuel required for stationkeeping is very small, months of potential stationkeeping time could be wasted if the ejection from orbital position occurs too early. On the other hand, if the decision is made too late, there may be insufficient fuel to cause the satellite to be removed from its orbital position, and the orbital position it occupies may not be usable for a replacement satellite. As a practical matter, the customer specifying the satellite to be manufactured may contractually require that a predetermined life be achieved.
Communication satellite weight is tightly controlled in order to provide the largest possible load of fuel for stationkeeping, thereby obtaining the longest possible useful life. For a satellite with 24 transponders which are polarization-multiplexed to provide 48 effective communication channels, the value of additional usable fuel may be several million dollars per additional month of useful life.
Thus, it is very advantageous to be able to determine the amount of remaining fuel. The measurement, however, is rendered complex because under weightless conditions, the fuel spreads through the tank. In order to prevent the fuel from forming into balls spread throughout the tank, the tanks include one or more thin circumferential bands near the inner surface of the tank. These bands capture the central bubble which tends to form, holding the remaining fuel against the outer walls and the bands.
A technique for using lightweight open electromagnetic transmission lines associated with the circumferential bands is described in U.S. patent application Ser. No. 07/285,699, filed Dec. 16, 1988 in the name of Hubert et al. That scheme requires the ability to apply electromagnetic signals to the interior of the fuel tank of the spacecraft, which undesirably may require one or more electrically insulated connections through the side of the fuel tank. An improved method and apparatus for gauging the amount of liquid or fuel in a storage tank is desired.